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Desalination of seawater has become a necessity, but it has to be done right.

Desalination of seawater has become a necessity, but it has to be done right.

As any globe will reveal, there's no shortage of water on Earth. Unfortunately, over 97 percent of it is too salty for us humans to drink, and only a tiny fraction of what remains is in the rivers, lakes, and groundwater that we're able to easily access.

In much of the world, these freshwater supplies are growing scarce, and competition for these resources promises to be one of the hot-button geopolitical challenges of the next 50 years and beyond. As climate change worsens droughts, accelerates desertification, and whittles away glaciers (the water towers providing life to so much of the world), it's no wonder that some experts are looking towards that enormous pool of salty water for a drink.

It's not a novel idea. Nearly 50 years ago President John F. Kennedy noted, "If we could ever competitively, at a cheap rate, get fresh water from salt water, that it would be in the long-range interests of humanity which would really dwarf any other scientific accomplishment."

About 2,300 years before Kennedy said that, Aristotle was already experimenting with the idea. Since then, desalination-or the process of removing salts from ocean or brackish water-has been proven possible, and employed in some form for ages. Around 200 AD, sailors boiled seawater and captured the salt-free evaporation when they ran out of drinking water supplies. This "thermal desalination" process can be scaled, but the costs are, for most, prohibitively high; most of the larger-scaled projects that took root were in the oil-rich and water-poor Middle East.

In the past couple of decades, though, a more promising, scalable solution has surfaced-reverse osmosis. Bear with me as I revisit high school chemistry. Take a semi-permeable membrane that water molecules can travel through, but not larger sediments like salt. Put very salty water on one side and less salty water on the other, and water will travel through towards the salty side until the concentrations are even. That's osmosis. Alternately, apply pressure to the saltier side, and water flows through the membrane, but the salt gets stuck. That's reverse osmosis, and the result is fresh water. And that's how most modern day desalination plants work.

Today, there are over 13,000 desalination plants around the world, with a collective capacity to produce about 14 trillion billion gallons of drinkable water every day. Sounds like a lot, but it's only about 0.5 percent of global demand. There are, however, many more in the works, particularly around large coastal cities in areas more vulnerable to drought or desertification. Parched Australia is a global leader, and an increasingly desperate California is getting serious about the technology. One plant planned for the San Diego area, for example, would churn out 50 million gallons per day, a drought-proof freshwater supply for about 300,000 people.

The upfront costs of building the plants are considerable-San Diego's Poseidon Plant is budgeted at $300 million; Melbourne is fixing to spend $2.9 billion on one that'd be amongst the world's largest-but after they're built, the chief expense is the energy it takes to push the seawater through the membranes.

Then there are the environmental costs, which are slowing down the approval processes in regulation-heavy places like California. As ocean water gets sucked into the system, aquatic organisms can get sucked up with it. Then, besides drinking water, there's the other byproduct of the process-very salty, and often hot, brine, which if released straight back into the ocean can create dead zones, worsening a problem already plaguing many coastal cities.

Both these problems can be addressed, albeit at some added expense. Sucking up seawater from beneath the sandy ocean floor avoids capturing unlucky creatures, and letting the brine mix with ocean water for awhile-as the Poseidon project is promising-before discharging it will prevent the dead zones.

But the chief environmental concern is certainly the energy it takes to run the system. There's a perverse logic in burning fossil fuels to make up for a shortage of freshwater-essentially worsening the problem you're trying to solve.

Of course, we can look to the wind and sun to power the desalination process. Offshore wind turbines make a lot of sense for plants that need to be located on the coast. Concentrated solar power could also do the trick. (Here's a study (pdf) that makes a very strong case for CSP powering desalination.)

The tough reality of the world's increasingly dire water crisis means that desalination isn't merely an option, but a necessity. The only sensible way to power these processes-without further contributing to one of the main causes of the freshwater shortages-is to do it without greenhouse gas emissions. Without exception, desalination needs to be coupled with clean energy.